What do I do with a fraud check when it wasnt my fault?

ok so i recieved a check in the mail from Accion Medical Christiana for 50.00 dollars. and I deposited the check into my account and it went thru. it was in my account for a couple days without even touching the money. and then of course i withdrew the money.now its been about almost 3 weeks and now im -50.00 in my account all of a sudden today. they told me that the charity company did not have any funds in the account. I didnt know it was a bad check(fraud). it was a charity that mailed me the check and it was a grant. and I bank with National City. Will i have to pay them back? what should I do?

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